The one unit I have seen is a modified rowing machine mounted out in the ocean perpendicular to the surface of the ocean. The handle/chain is connected to a floating weight, as the waves oscillate, the wieght goes up and down, pulling the chain up and down, driving the rowing machine. Instead of a resistance fan(used for exercise), the rowing machine is outfitted with a generator.

You can build one of these machines, don’t be afraid to take the first step.